The present invention relates to a nonaqueous electrolyte secondary battery.
Recently, a lithium ion secondary battery has been put on the market as a nonaqueous electrolyte secondary battery for portable apparatuses such as portable phones. This battery uses lithium cobalt oxide (e.g., LiCoO2) as a positive electrode active material, a graphite material or carbonaceous material as a negative electrode active material, an organic solvent having a lithium salt dissolved as a nonaqueous electrolyte, and a porous film as a separator. A nonaqueous solvent having a low viscosity and a low boiling point is used as a solvent of the electrolyte. For example, Japanese Patent Disclosure (Kokai) No. 4-14769 discloses a nonaqueous electrolyte secondary battery comprising an electrolyte including a mixed solvent consisting essentially of propylene carbonate, ethylene carbonate and xcex3-butyrolactone, the xcex3-butyrolactone occupying 10 to 50% by volume of the entire solvent mixture. On the other hand, Japanese Patent Disclosure (Kokai) No. 11-97062 discloses a nonaqueous electrolyte secondary battery using a nonaqueous electrolyte prepared by dissolving lithium borofluoride (LiBF4) in a solvent consisting of 100% by volume of xcex3-butyrolactone.
It is of high importance nowadays to decrease the thickness of the secondary battery in accordance with decrease in the thickness of the portable apparatus. In order to decrease the thickness of the secondary battery, it is necessary to decrease the thickness of the jacket housing the positive electrode, the negative electrode, the separator, and the nonaqueous electrolyte. However, in the nonaqueous secondary battery provided with a nonaqueous electrolyte including a mixed solvent containing 10 to 50% by volume of xcex3-butyrolactone, a reaction takes place between the positive electrode and the nonaqueous electrolyte when the secondary battery is stored at such a high temperature as 60xc2x0 C. or higher. As a result, the nonaqueous electrolyte is decomposed by oxidation to generate a gaseous material. And also, a gas material is likely to be generated from the negative electrode during the initial charging. What should be noted is that, if the thickness of the jacket is decreased, the jacket is swollen by the gas generation so as to be deformed. If the jacket is deformed, an electronic equipment cannot be housed in the battery. Alternatively, malfunction of the electronic equipment tends to be invited.
In the nonaqueous electrolyte secondary battery, it is also important to further improve the large discharge characteristics and the charge-discharge cycle characteristics.
An object of the present invention is to provide a nonaqueous electrolyte secondary battery that permits suppressing the gas generation during storage of the battery under high temperatures so as to prevent a jacket from being swollen and also permits improving the large discharge characteristics and the charge-discharge cycle life.
According to the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising an electrode group including a positive electrode, a negative electrode containing a material for absorbing-desorbing lithium ions, and a separator arranged between the positive electrode and the negative electrode, a nonaqueous electrolyte impregnated in the electrode group and including a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent, and a jacket for housing the electrode group and having a thickness of 0.3 mm or less, wherein the nonaqueous solvent contains xcex3-butyrolactone in an amount larger than 50% by volume and not larger than 95% by volume based on the total amount of the nonaqueous solvent.
According to the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising an electrode group including a positive electrode, a negative electrode containing a material for absorbing-desorbing lithium ions, and a separator arranged between the positive electrode and the negative electrode, a nonaqueous electrolyte impregnated in the electrode group and including a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent, and a jacket for housing the electrode group, the jacket being made of a sheet having a thickness of 0.5 mm or less including a resin layer, wherein the nonaqueous solvent contains xcex3-butyrolactone in an amount larger than 50% by volume and not larger than 95% by volume based on the total amount of the nonaqueous solvent.
Further, according to the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising an electrode group including a positive electrode including a collector and a positive electrode layer formed on one or both surfaces of the collector and containing an active material, a negative electrode including a collector and a negative electrode layer formed on one or both surfaces of the collector and containing a material for absorbing-desorbing lithium ions, and a separator arranged between the positive electrode and the negative electrode, a nonaqueous electrolyte impregnated in the electrode group and including a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent, and a jacket for housing the electrode group and having a thickness of 0.3 mm or less, wherein the positive electrode layer has a porosity lower than that of the negative electrode layer, the positive electrode layer has a thickness of 10 to 100 xcexcm, and the nonaqueous solvent contains 40 to 95% by volume of xcex3-butyrolactone based on the total amount of the nonaqueous solvent.
Further, according to the present invention, there is provided a nonaqueous electrolyte secondary battery, comprising an electrode group including a positive electrode including a collector and a positive electrode layer formed on one or both surfaces of the collector and containing an active material, a negative electrode including a collector and a negative electrode layer formed on one or both surfaces of the collector and containing a material for absorbing-desorbing lithium ions, and a separator arranged between the positive electrode and the negative electrode, a nonaqueous electrolyte impregnated in the electrode group and including a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent, and a jacket for housing the electrode group, the jacket being made of a sheet having a thickness of 0.5 mm or less including a resin layer, wherein the positive electrode layer has a porosity lower than that of the negative electrode layer, the positive electrode layer has a thickness of 10 to 100 xcexcm, and the nonaqueous solvent contains 40 to 95% by volume of xcex3-butyrolactone based on the total amount of the nonaqueous solvent.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.